1. Field of the Invention
The present invention relates to a microwave oven which is capable of shortening defrosting time and improving the grade of finishing of the defrosted target by improving distribution of temperatures during the defrosting process.
2. Description of the Related Art
A conventional technique will now be described with reference to FIG. 71. Reference numeral 1 represents a magnetron for generating microwaves. Microwaves generated by the magnetron 1 dielectrically heat a target 7 which must be heated so that the target is cooked with heat. A waveguide 2 guides, into a heating chamber, the microwaves generated by the magnetron 1. The target 7 which must be heated is accommodated in a heating chamber 6 so that the target is cooked with heat in the heating chamber 6.
The output of the microwaves is regularly turned on or off in accordance with the state, for example, the weight of the target 7 which must be heated so that dielectrical heating is performed for the purpose of heating and defrosting the target.
The conventional control of microwaves raises the following problems.
When a target which has been frozen in a freezing chamber or the like in a freezer is defrosted with heat generated by a dielectric phenomenon, the overall body of the target which must be heated is frozen. That is, internal water is in a solid state when irradiation with microwaves is started. When a target which must be heated is irradiated with microwaves in the state in which internal water in the solid state exists in the target which must be heated, the temperatures of the inside portions of the target which must be heated are substantially uniformly raised. When phase shift takes place causing internal water to be converted from a solid body into liquid, rise in the temperature in the internal portions, however, varies considerably because of variations of compositions of materials in the target which must be heated and the shape of the target.
That is, the variation in the components of the target which must be heated causes molar depression of the freezing point to take place variably in each portion in the target which must be heated. As a result, the defrosting temperature varies. The phase shift from the solid state to liquid requires heat of melting. When the energy of microwaves is used as the heat of melting, the temperature of the target which must be heated is not raised.
If the target which must be heated and which in in the above-mentioned state is irradiated with microwaves similarly to the state in which internal water is in the solid state, defrosted portions and non-defrosted portions appear in the target which must be heated. As a result, portions, in which solid state internal water exists and portion, in which internal water has been formed into liquid, nonuniformly exist. If the irradiation with microwaves is continued in the foregoing state, the temperatures of the portions of the target changed into water are rapidly raised as compared with the portions of the target in which internal water remains in the ice state because water in the liquid state can easily dielectrically be heated by microwaves by thousands times as compared with solid water, that is, ice. Therefore, in extreme cases, continuation of irradiation with microwaves causes the portions immediately after defrosted and boiled portions to exist together in the target which must be heated.
To prevent the nonuniform result of the defrosting process, the conventional control of microwaves has been performed in such a thanner that microwaves are applied with full power in a case where the overall body of water in the target which must be heated is in the solid state. Moreover, when the temperature has approached a level at which the phase is shifted, that is, when the temperature in the target which must be heated has approached zero degree, the control is performed in accordance with an on/off pattern which is a regular pattern in terms of time. As an alternatively to this, a method of adjustment has been employed in which the output of microwaves is rapidly reduced so that the phase shift is completed slowly. Another adjustment has been employed in such a manner that water in the target, which must be heated, is uniformly converted into water having a uniform temperature level. As a result, a considerably long time takes place for the microwave oven to complete the defrosting process.
An optimum cycle of the on/off pattern is determined for collectively shifting the phase of the target which must be heated as a result of experiments of cooking operations. The foregoing adjustment, however, is a too delicate operation. Therefore, the optimum cycle cannot easily be determined.
Thus, partial dispersion inevitably takes place when the phase of water is changed in the target which must be heated. As a result, the temperatures of the portions in which ice has been changed to water are rapidly raised. Thus, the temperatures in the target which must be heated are made to be nonuniform. Moreover, the degree of nonuniform temperatures realized after the defrosting process varies depending upon the state of the target which must be heated, the structure of the microwave oven and the like. Thus, a constant state of defrosting cannot easily be realized.